I am looking to fill a spool on one of my reels with line of 3lb to 4lb breaking strain (0.16 – 0.18) for fishing the waggler, and want something that sinks well. I would prefer something that I can fill the spool with rather than have backing, so something on 300m spools would be good. I have tried a number of lines but none of them seem to sink that well, I appreciate you can treat the line (I have used some old Maver line sink) but the effect is fairly temporary and its a pain doing it multiple times.

All solid nylon lines will sink as nylon is heavier than water. The reason most float (virtually all) when new is, apart from surface tension etc, that they have a silicone type coating on their surface to help when spooling in bulk etc in the factory. Wash it off and they sink. If you want very heavy solid mono then go for fluorocarbon as it’s much heavier than nylon.